Von Meyer proposed to separate this order into two groups, one with two phalanges in the wing-finger, of which Ornithopterus is the only example, forming his Diathri; while the other group, Tetrathri, or those "with four fingers, comprised all other Pterosaurians. The Tetrathri he again subdivided, following out, as he states, the suggestion of Munster and Goldfuss, into _Dentirostres_ or such Pterodactyles as have the jaws furnished with teeth to their anterior termination; and the _Subulirostres_, or such as want teeth at the extremities of the jaws. To the former group he left the name Pterodactylus, and to the latter was given the name Rhamphorhynchus. Von Meyer says that he might easily have made a few more species, as will be evident to those who inspect his plates, but he "believes that the students of living animals go too far in their tendency to subdivide:" a fancy that, if indulged in by Palæontologists, would have the effect of restoring the old Linnæan groups; and a complaint which, although often heard, has usually come from those who do not readily discern and appraise classificational characters. In Palæontology genera are sometimes co-extensive with orders, while species often mean genera. It may be wearisome to the collector to be lured on to follow the devious ways of a science, but Palæontology, the source whence the mysteries of existing nature must unravel their meaning, is the handmaid of all nature's truths which have been buried in evolving the existing creation; and a duty devolves upon Palæontologists to make the past an inseparable part of the present, by applying to the two the same scientific method. A year previous to the formation of Owen's Pterosauria, Bonaparte named the Order Ornithosaurii, and divided it into a family--Pterodactylæ, and a sub-family Pterodactylinæ. Fitzinger (_Systema Reptilium_, 1843) also used the same ordinal name, and recognized three genera-- _Pachyrhamphus_, of which the type is Pterodactylus crassirostris (Gold.). _Pterodactylus_, with the type P. longirostris (Cuv.). And _Ornithocephalus_, with the type O. brevirostris (Sömm.). These and other attempts at classification all endeavour to subdivide Ornithosaurians by the head or by the tail. Other characters for primary divisions may be obtained from the pelvis. In the majority of German Pterodactyles the ilium extends for a long distance in front of the os pubis, and only for a very short distance behind the large ischium; and the small pubis from its anterior margin gives attachment to a large prepubic bone, which resembles in form the os pubis of the Crocodile[Z], and is unlike that of the Monotreme. These appear to include the long-legged animals with short tails, at present called Pterodactyles, and form a well-marked family or order. [Footnote Z: Prof. Haughton, from a study of the bones and muscles, came to the conclusion that the pubic bones of Crocodiles are the marsupial bones.]